This invention relates to valve-gated injection molding systems and more particularly to an improved nozzle seal extending across the air space from the heater cast to form the gate.
As shown in the applicant's U.S. Pat. No. 4,043,740 which issued Aug. 23, 1977, it is known to provide a nozzle seal which is seated in the nozzle portion of the heater cast and extends across the air space to abut on the cavity plate around the gate. While this does prevent the escape of pressurized melt into the air space and does increase heat transfer to the gate area, there are remaining problems which are not overcome when certain materials are used for certain applications. Normally, the valve pin is not closed for 3-4 seconds after the cavity is filled to allow for shrinkage. However, during this period a thin layer or film of melt solidifies on the inside of the gate and then when the valve pin is closed, is applied considerable pressure to this solidified material. This results in certain materials such as polyester crystalizing when the pressure is applied. During the next cycle when the valve pin is opened, this crystalline material passes through the gate into the cavity which produces flaws in the molded product. This is particularly unsatisfactory when molding polyethylene terephthalate thermoplastic polyester preforms to be blow molded into bottles because the crystalline material which is formed is milky and cannot be blown. Another remaining problem with the previous system is that a bubble may be formed on the product when the mold is opened and the product is pulled away from the gate.
While it is known to provide externally heated heater casts or nozzles which extend directly to the cavity through a gate aperture in the cavity plate, this arrangement has the disadvantage of resulting in excessive heat loss through the direct contact between the cooled cavity plate and the heated nozzle which must be formed of a relatively conductive material.
In order to overcome these problems, it is necessary to provide for the transfer of heat right into the gate area adjacent the cavity. However, this must be done without reducing the effective cooling of the adjacent portions of the cavity plate. As will be appreciated, the more heat that is conducted to the area, the more important the effectiveness of the cooling becomes. Otherwise, the product will not set quickly enough to provide efficient operation of the system.